The present disclosure relates to an unmanned underwater vehicle for deep water conditions. The disclosure also relates to a method of controlling a hydraulic system in such a vehicle.
A hydraulic system uses a pressurized hydraulic fluid, e.g. oil, to power hydraulic tools. The hydraulic system typically comprises a hydraulic pump driven by a motor to pressurize the hydraulic fluid. The pressurized fluid is guided via a hydraulic circuit to the tools. The hydraulic circuit may comprise valves, filters, piping etcetera to guide and control the system. A hydraulic tool may comprise an actuator such as a hydraulic motor or cylinder to actuate, i.e. mechanically drive, the machinery. Mechanical operation of the tools can be controlled e.g. by opening or closing hydraulic valves in the circuit between the pump and the tool.
Hydraulic systems can be advantageously used under water because power can be conveniently transmitted without electrical connections. However, when operating in deep water conditions, the hydraulic fluid may be affected by cold surroundings and/or high pressure of the water. In particular, when the hydraulic fluid cools down, the fluid may become thick and difficult to move through the hydraulic circuit, especially if the fluid has become stagnant when a tool is not used for some time.
One solution to alleviate thickening at cold surroundings is to replace the hydraulic fluid for fluid having lower viscosity. However, using low viscosity oil may affect operation at relatively high temperature conditions. Another solution for alleviating thickening may be to heat the fluid. However, the heating mechanisms available underwater such as, e.g., electrical heating under water may consume too much power and could present failures, such as short circuits.
Accordingly, it is desired to improve hydraulic systems and their operation for use in cold surroundings, in particular in an unmanned underwater vehicle for deep water conditions.